1. Field of the Invention
The present invention relates to inkjet recording techniques in which recording is performed by discharging ink toward a recording medium from a long recording head (hereafter called a head assembly) obtained by connecting a plurality of head chips, each having multiple nozzles. More specifically, the present invention relates to an inkjet recording technique in which an image is recorded on a recording medium with a single scan of a head assembly relative to the recording medium (single-path method). The head assembly is obtained by disposing a plurality of relatively short head chips, each having multiple nozzles arranged therein, in the arrangement direction of the nozzles with high accuracy.
2. Description of the Related Art
In printers, printing apparatuses used in copy machines or the like, and printing apparatuses used as output apparatuses in workstations or complex electronic systems including computers and word processors, images (including characters and symbols) are printed on printing media, such as paper or thin plastic plates, on the basis of print information. The printing methods of these printing apparatuses are classified into an inkjet method, a wire-dot method, a thermal method, a laser beam method, etc.
An inkjet recording apparatus using the inkjet method is disclosed in, for example, Japanese Patent Laid-Open No. 8-300644.
Among various types of printing methods that are presently known, a typical printing apparatus using the inkjet printing method is a serial printing apparatus which performs printing by repeatedly moving a recording head having multiple nozzles arranged therein in a direction different from the arrangement direction of the nozzles. In the serial printing apparatus (also called a serial-scan printing apparatus), the entire region of a recording medium is printed on by repeating a main-scan recording step of forming an image by moving a print unit (recording head) along the recording medium in a main-scanning direction and a sub-scanning step of moving the recording medium by a predetermined distance each time a single scan is finished.
In such an inkjet printing apparatus (recording apparatus), normally, a band-shaped image region (hereafter called a band) is formed with a single scan, and ink spreads depending on the material and the surface state of the recording medium. Accordingly, irregular image regions called “connection lines” are formed in boundary regions between the bands.
As a recording method for eliminating the above-described irregular image regions, a multi-path method is known in which a single band is recorded with multiple scans. However, in the multi-path method, the number of times a recording head is moved relative to a recording medium is increased and the time required for recording the entire region of the recording medium is increased accordingly. As a result, the recording speed is reduced.
The connection lines between the bands can be eliminated without increasing the time for recording on the recording medium by using a recording apparatus including a long recording head in which nozzles are arranged over a distance longer than a dimension of the recording area. As an example of such an apparatus, a full-line (full multi) recording apparatus is known in which a recording head (full-line head or full multi head) having a length corresponding to the entire (or substantially entire) width of a recording medium is moved relative to the recording medium along the length of the recording medium. In the full-line recording apparatus, image printing is completed with a single scan, and the bands are not formed unlike the serial printing apparatus. Accordingly, in the full-line recording apparatuses, the above-described irregular image regions are not formed between the adjacent bands.
However, when the above-described long head is manufactured, it is extremely difficult to form the nozzles and print elements, such as piezoelectric elements and heating resistance elements, over the entire width of the recording area without any defects. For example, in full multi printers used in offices or the like to output photographic images on large paper, about 14,000 nozzles are required to print on A3-sized paper with a resolution of 1,200 dpi (recording width is about 280 mm). It is difficult to form inkjet print elements corresponding to such a large number of nozzles without any defects in view of the manufacturing process thereof. Even if it is possible to manufacture such a print head, the percentage of defects is high and extremely high costs are incurred.
Accordingly, inkjet recording apparatuses having the structure of line printers including full multi print heads have been suggested. For example, Japanese Patent Laid-Open No. 3-54056 discloses a recording apparatus using a head obtained by connecting a plurality of head chips (also called nozzle chips).
FIGS. 3 and 4 are schematic diagrams showing examples of heads obtained by connecting a plurality of head chips (also called nozzle chips). Multiple nozzles are arranged in each of the head chips. The head chips are linearly disposed in the arrangement direction of the nozzles in the example shown in FIG. 3, and are disposed in a staggered pattern in the example of FIG. 4.
The above-described head (hereafter called a head assembly) is obtained by arranging a plurality of short, relatively inexpensive head chips that are commonly used in serial recording apparatuses with high accuracy. The number of nozzles formed in a single head chip is smaller than that in a single long head, and therefore the percentage that defective nozzles are present in the head chip is low. Thus, the percentage of defects is lower than that in the case of manufacturing a head having an integral structure with a plurality of nozzles arranged therein. In addition, only the head chips having defects are treated as defective parts, and therefore the manufacturing cost of the head is reduced.
Accordingly, a full-line recording apparatus can be relatively easily manufactured when the head assembly structured as described above is used as a full-line head that records over the entire width of the recording medium. In addition, when the head assembly is used in a serial recording apparatus, the width of a band recorded with a single scan is increased and the number of boundaries between the bands appearing in the image recorded on a single recording medium is reduced accordingly. Therefore, the irregularity of the image is reduced and the recording speed is increased at the same time.
However, when the head assemblies structured as shown in FIGS. 3 and 4 are used, the amount of heat generation varies between the chips due to the structure thereof, and accordingly the temperature varies between the chips.
On the other hand, a bubble jet recording method in which ink is discharged using heat is known as an example of the inkjet method. In the bubble jet recording method, bubbles are generated in the ink by heating the ink, and the ink is discharged though the nozzles by the pressure applied when the bubbles are generated. The above-described problem of variation in heat generation is particularly crucial in the bubble jet recording method.
With respect to the temperature distribution in each head chip used in the above-described bubble jet method or the heat transfer method, the head chip is normally formed on a silicon substrate, which has very high thermal conductivity, by a semiconductor manufacturing process or photolithography. In addition, the size of each head chip (short chip) included in a full line head is about 0.5 inches. Under these conditions, the temperature distribution in each chip becomes uniform in a relatively short time. However, in the head assembly including a plurality of head chips, the head chips are formed independently of each other and are separated from each other in the example shown in FIG. 4. Therefore, heat is transmitted between the head chips via a base plate composed of, for example, alumina, carbon, aluminum metals, etc., to which the head chips are adhered, and the temperature variation between the head chips is too large to be ignored when the head assembly is used. This problem does not occur when the recording head having an integral structure with all of the nozzles formed therein is used.
In the inkjet recording head, the volume of a single ink drop discharged from a nozzle generally varies depending on the temperature, and the difference in the volume of the ink drop appears in the image on the recording medium as a density difference. Accordingly, the temperature variation between the head chips appears as the density variation between the image regions corresponding to the head chips, and is visualized as band-shaped regions in the image.
In the case in which recording is performed using a serial scan recording apparatus including the head assembly by a single-path method in which an image is recorded with a single scan, head chips that are most distant from each other in the head assembly form an image region at the boundary between the bands. Since the head chips are influenced by the distance therebetween with regard to the heat diffusion in the head, a large density difference is generated in the region between the bands.